Commutating structure for DC machines

ABSTRACT

A DC permanent magnet machine having stator and rotor assemblies of the &#34;inside out&#34; design. The stator is provided with a plurality of electrically energizable poles having windings which are electrically coupled to associated commutator bars forming an annular array. The rotor is provided with a plurality of permanent magnet poles equal in number to the poles provided in the stator. A plurality of roller contact assemblies mounted upon the rotor serve to progressively and sequentially couple the commutator bars to brush rings and through brushes to opposite polarities of a DC energizing source, thereby sequentially providing electrical power to associated coils of the stator winding. The interaction of the magnetic field created by the energized stator coils and the magnetic field of the permanent magnets in the rotor assembly causes rotation of the rotor which, in turn, rotates the roller contacts to continuously and progressively switch the DC source to succeeding stator coils. The roller contacts are arranged in an assembly which greatly enhances the dynamic stability of the rollers despite uneven wear of the rollers caused by long, continued use. The commutator bar array is kept free of dirt and/or conductive particles by the blower action created by openings in the rotating roller contacts which move air therethrough by centrifugal force. Alternatively, fan blades on the rotor may be used to create the blower action. If desired, both techniques may be used. The configuration of the commutator array protects adjacent bars against short-circuiting by conductive particles collecting therebetween.

This application is a continuation-in-part of my application Ser. No.471,313 filed May 20, 1974, now U.S. Pat. No. 3,876,892, which in turnis a continuation-in-part of my application Ser. No. 383,883 filed July30, 1973, now U.S. Pat. No. 3,819,964.

BACKGROUND OF THE INVENTION

Conventional DC motors typically utilize a multipole permanent magnetstator and a rotor having a plurality of energizable conductors arrangedabout the surface thereof and parallel to the axis of rotation. Pairs ofsaid conductors are electrically energized by rotating commutator barsprovided on the rotor, which commutator bars are electrically energizedby a DC source through stationary commutator brushes which make wipingengagement with the commutator bar array. Interaction of the statorpermanent magnet fields with the magnetic fields created by DCenergization of the rotor conductors provides for rotation of the rotor,whereby relative rotation of the commutator bars and commutator brushescontinuously changes the electrical connections between the DC sourceand the array of conductors in the rotor. Significant wearing of thecommutator bars is caused both by the sliding friction of the brushesand the burning action of the commutating current, thereby reducing theuseful operating life of the motor.

The above disadvantages, as well as the fact that conventional DC motorshave high inertia, has led to the development of the inside out motordesign in which the multi-pole rotor is provided with permanent magnetmembers and the stator is provided with an equal number of poles whosewindings are energized by the DC source. This design provides a rotorwith lower inertia for a given peak torque, and a stator having greatercopper volume and better heat dissipation as compared with conventionalDC motor designs. Thus, the inside out motor design has a highercontinuous rating in contrast to conventional DC motors of the same sizeand weight.

The problems of commutation in motors of the inside out type have led tothe development of a DC brushless type motor which employs electronicamplifiers and other solid state circuit elements to provide thenecessary commutation. The electronic amplifiers and circuit elementsrequired for proper switching of power to the stator windings togenerate the rotating field add significant cost and weight to themotor. The solid state switching circuitry also increases motor"cogging" which occurs during low speed motor operation.

The numerous problems and disadvantages encountered in DC motors of boththe conventional and inside out design have, in turn, led to thedevelopment of the design described in U.S. Pat. No. 3,819,964 which ischaracterized by providing novel electromechanical switching techniquesfor commutating the motor windings.

In a preferred embodiment of the invention disclosed in U.S. Pat. No.3,819,964, the stator assembly is provided with first and second annularconductive rings connected to opposite polarities of a DC source and anannular array of commutator bars disposed proximate thereto. The rotorassembly is provided with a plurality of roller contacts which revolvewith the rotation of the rotor shaft to simultaneously couple theopposite terminals of the stator coils to the opposite polarities of theDC source so as to progressively energize the stator coils, the magneticfields of which interact with the rotating magnetic fields of the rotorpermanent magnets to effect rotor rotation.

The commutating technique of the above design exemplified by U.S. Pat.No. 3,819,964 employs a "rigid" roller concept in which the rollercontacts are used to bridge between the conductive rings and thecommutator bars. These rollers are also mechanically tied together. Inthis approach, the associated rollers must operate at the same speed.Any factor which results in the production of differing roller or ringdiameters or any other condition which would cause one of the rollers tooperate at a different speed would impose upon its associated roller therequirement that it must slip with respect to the other. Also, if afterlong, continued use one of the rollers wears at a rate different fromits associated roller causing its diameter to change, slippage willoccur. It has also been found that a structure mechanically tying tworollers together introduces dynamic instability in that the moment ofinertia of the roller assembly about an axis at right angles to the axisof rotation is quite high and any bounce or eccentricity is greatlymagnified at high rotating speeds which can cause the rollers to pullaway from the contacting surfaces.

In addition, dust and/or conductive particles developed by wearing ofthe moving parts and/or introduced from the surrounding environment maysettle and collect upon the stator mounted (and hence stationary)commutator array resulting in undesirable and even harmfulshort-circuiting of adjacent commutator bars.

The rotor permanent magnet assembly also has the disadvantage of beingsusceptible to demagnetization. Also, for DC machine applicationsrequiring high output shaft velocities it is desirable to operate theroller contacts employed for commutation at lower angular velocities.

The motor, in one preferred embodiment, is of a modular designcomprising a hermetically sealed housing containing the stator coils androtor permanent magnet structure. The commutation structure is coupledthereto and is enclosed in separate accessible housing.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the present invention conductive rollers are providedwhich incorporate the dual functions of providing rolling contact withthe commutator bar array and creating a blower action which serves tofree the array of unwanted dust and/or conductive particles. Thecommutator array is designed to provide a gap between adjacent barswhich allows for collection of particles while simultaneously preventingshort-circuiting.

Each roller is independently spring loaded against its mating surfaceand therefore can operate independently of the other thereby greatlyenhancing the dynamic stability of the roller assemblies. The rollercontact spring arrangement conducts current through a flexible conductorfrom a selected one of two independent brush rings mounted upon therotor. Spring loaded brushes, which form part of the stator structure,wipingly engage the rotor brush rings and couple the opposite polaritiesof a DC source thereto.

The rollers have openings in their side walls which communicate withopenings in their cylindrical surface. The centrifugal force created byrotation of the rollers during machine operation causes air to be drawninto the side openings and forced out of the openings in the cylindricalsurface thereby creating a blower action to rid the commutator bar arrayof dust and/or conductive particles.

Either as an alternative, or in addition to the blower action providedby the rollers, the rotor may be fitted with fan blades aligned adjacentto the stator commutator array to keep the array free of foreign matter.

The commutator bars are also arranged in a "see-through" manner wherebygaps are provided between adjacent bars. The gaps narrow towards theirbases wherein the sidewalls of each gap are respectively one sidesurface of a commutator bar and one side surface of an insulationmaterial interposed between the above mentioned bar and the nextadjacent bar to provide a region in the bottom of the gap which is freeto collect dust and/or conductive particles without danger ofshort-circuiting the commutator bars.

The rotor may comprise a permanent magnet assembly having laminated ironpole pieces surrounded by magnetic members which enhance impedancematching, increase flux density in the air gap and resistdemagnetization. In high speed DC machine applications the rotor mountedroller contacts may be of increased diameter and rollingly engage theouter peripheries of the stator mounted commutator bars.

The DC machine may be of a modular design, comprising a hermeticallysealed housing containing the stator windings and rotor permanent magnetassembly. A separate accessible housing then contains the commutationassembly which may be either magnetically or mechanically coupled to therotor assembly drive shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a DC machine of the inside-out type whichembodies the principles of the present invention;

FIG. 2 is a partially sectionalized detail view of the roller contactassembly of FIG. 1;

FIG. 2a is a partial end view of the roller contact assembly of FIG. 2;

FIG. 3 is an end view of the rotor permanent magnet assembly of FIG. 1;

FIG. 3a is a side view of the permanent magnet assembly of FIGS. 1 and 3illustrating the manner of assembly;

FIG. 4 is a partially sectionalized end view of a molded commutatorarray embodying the principles of this invention;

FIG. 4a is a partial front view of the commutator bar array of FIG. 4;

FIG. 4b shows a detailed sectional view of one bar of the commutator bararray of FIG. 4;

FIG. 5 is a simplified diagrammatic view of a DC machine of modulardesign embodying the principles of the present invention; and

FIG. 6 is a simplified diagrammatic view of a DC machine for use in highspeed operation and which embodies the principles of the presentinvention.

DETAILED DESCRIPTION

FIG. 1 shows a preferred embodiment 10 of the invention which iscomprised of housing members 11 and 12 which are each provided withopenings 11a and 12a for receiving bearings 13 and 14 which surround arotatably mounted rotor shaft 15. The inner ends of housing members 11and 12 are hollow and are contoured or otherwise formed to receive andsupport the stator and rotor assemblies.

The stator assembly is comprised of a laminated core 16 formed ofindividual laminations 16a. The stator winding is comprised of aplurality of coils 17 (not shown in detail for purposes of simplicity)which, when energized, create magnetic fields in the stator core whichinteract with the magnetic fields set up in the rotor assembly to effectrotor rotation.

Housing member 12 is further adapted to receive the commutator assembly18 which includes a plurality of commutator bars 19 mounted in radialfashion (see FIGS. 4-4b) within annular-shaped molded insulatingmaterial 42. Selected ones of the commutator bars are electricallyconnected to the end terminals of associated stator coils. As shown inFIG. 4a, the commutator bars 19 are skewed at a small angle θ so that aroller contact moving left to right with respect to FIG. 4a will effecta make-before-break contact with the commutator bars. In addition,skewing the commutator bars provides a smooth rolling surface for theroller contact so that it will mate smoothly with the inside surface ofthe commutator.

As shown in FIG. 1, the rotor assembly comprises a permanent magnetarray secured to shaft 15. The outer periphery of the permanent magnetarray lies a small, spaced distance from the interior periphery of thestator core 16 to form a hollow, annular-shaped air gap G therebetween.

Referring now to FIGS. 2-2a, the details of roller contact assemblies 22will be explained. Unitary sleeve 20 is mounted on rotor shaft 15 (seealso FIG. 1) and has an annular recess 20a for positioning floatingroller platform 30 which is resiliently mounted to sleeve 20 by springs31 and 31'. Support arms 30a and 30b respectively support roller shaft32 and contact 33. Spring 34 urges roller 35 towards contact 33. In theevent of excessive wear, that portion of roller 35 bearing againstcontact 33 may be fitted with a plug of highly conductive, lowresistance material. The arms 30a and 30b are slidably supported by theslots 20c provided in upright supports 20b arranged at spaced intervalsabout sleeve 20.

Conductive roller 35 is provided with hollow elongated passageways 35aparallel to the rotating axis and which extend to both sidewalls of theroller. The passageways communicate with radially aligned passageways35b which open into the cylindrical surface of the roller.

Fan blades 36 are molded into sleeve 20 and are interspersed betweenadjacent pairs of rollers.

Sleeve 20 supports continuous brush rings 23 and 24, each of which iselectrically connected to selected ones of the roller assemblies 22 byflexible conductors 37, 37a. Brushes 25 and 26 are spring loaded andmounted upon the stator assembly. They engage brush rings 23 and 24respectively. The brushes are, in turn, connected to opposite polaritiesof a DC power source (not shown). While depicted in FIG. 1 as disposedto one side of the roller contact assemblies 22, brush rings 23 and 24may be positioned on opposite sides of the roller contact assemblies, ifdesired.

The rolling engagement between rollers 35 and the bars 19 of thecommutator array rotates the rollers creating a centrifugal force whichcauses air to be drawn into the side openings of passageways 35a andurged out of radial passageways 35b. The rapidly moving air blows dustand/or other conductive particles away from the commutator array. Blades36 serve the same function.

Rollers 35 of roller contact assemblies 22 progressively rollinglyengage the commutator bars 19 while brushes 25 and 26 wipingly engageconductive rings 23 and 24, whereby the electrical path extends from oneterminal of the DC source to brush 25, brush ring 23, flexible conductor37, contact 33, conducting roller 35 and the commutator bars 19. Theopposite polarity of the DC source is coupled to brush 26, brush ring24, flexible conductor 37a, a roller (spaced from roller 35 of FIG. 2)and commutator bars 19. The end terminals of the stator winding are thusprogressively energized and the magnetic field generated therebyinteracts with the magnetic field of the rotor permanent magnetstructure to sustain rotation.

The commutator bar array of FIGS. 4-4b contributes to the blower actionby providing gaps 40 between adjacent bars 19. Each commutator bar has aroller contact portion 19a and upright arms 19b and 19c. Arm 19b extendsinwardly at 19d to secure commutator bar 19 to insulating material 42.Terminals of the stator windings are connected to commutator bars 19 at19e. The bars 19 are embedded in an insulating material 42 which extendspartially into each gap 40 and engages one wall of each bar. Forexample, molded portion 40a engages one sidewall of bar 19' and isspaced from the adjacent sidewall of bar 19". Particles falling into gap40' and collecting in the bottom-most portion are prevented fromcreating an electrical path between bars 19' and 19" due to the presenceof molded insulating portion 40a. The blower action created by fanblades 38 and/or rollers 35 keep gaps 40' clear of particles.

FIGS. 3 and 3a show the rotor permanent magnet structure in greaterdetail. The permanent magnet structure comprises a Plurality oflaminated iron pole piece assemblies 44 each having individual polepieces 45 (see also FIG. 1) and each having an arcuate outer peripheryand radially aligned sides 44b and 44c. Each pole piece is provided withan opening 44e. The sidewalls of laminated assemblies 44 are embraced bysolid rectangular-shaped permanent magnets 46.

Rotor shaft 15 has a hexagonal-shaped cross section extending the lengthof the permanent magnet assembly and is preferably formed of a magneticmaterial such as, for example, soft iron. Elongated rectangular-shapedpermanent magnets 47 are positioned in pole pieces 45 and an associatedsurface 15a of rotor shaft 15. The magnets 46 are preferably rare-earthmagnets which resist demagnetization, provide better impedance matchingand serve to increase flux density across the air gap G (see FIG. 1).Magnets 47 are preferably Alnico-8 magnets. The magnet members 46, 47and 46 embrace pole pieces 45 and serve to concentrate the flux densityin the air gap G.

FIG. 3a shows the manner of assembly of the rotor permanent magnetstructure. End caps 49 and 50, rods 51 and fastening nuts 52 hold thepermanent magnet assembly together. Set screws 53 and 54 engage tappedopenings in collar portions 49a and 50a of the end caps 49 and 50 tolock the assembly to shaft 15.

FIG. 5 shows a DC machine 60 of modular design comprised of ahermetically sealed housing 61 having internally mounted bearings 62 and63 for rotatably mounting shaft 15. The permanent magnet assembly 23,which is preferably of the type shown in FIGS. 3 and 3a, is mounted uponrotor shaft 15.

The stator assembly has a laminated core 16 comprised of individuallaminations 16a. The stator winding is comprised of a plurality of coils17 (not shown in detail) which, when energized, create magnetic fieldsin the stator core which interact with the magnetic fields set up by therotor permanent magnet assembly to effect rotor rotation.

The end terminals 17a and 17b of the stator coils are led out of thehermetically sealed housing and terminate at a hermetically sealedterminal assembly 64 molded into side face 61a of housing 61.

A second housing 65 has molded or otherwise provided along one side wall65a a mating terminal assembly 66 which is releasably inserted intoterminal assembly 64. Housing 65 is provided with bearing assemblies 67and 68 for rotatably mounting shaft 69.

Roller contact assemblies such as, for example, 70 and 70' and brushrings 71 and 72 are mounted on sleeve 73 which encircles shaft 69.Stationary mounted brushes 74 and 75 are secured within housing 65 andrespectively wipingly engage rings 71 and 72. Opposite polarities of aDC source are electrically connected to brushes 74 and 75 by conductors76 and 77 which extend between brushes 74 and 75 and the exterior ofhousing 65. Flexible conductors 78 and 79 electrically connect brushrings 74 and 75 to spring loaded contacts 80 and 81 which engage rollercontacts 82 and 83 respectively.

Roller contacts 82 and 83 are preferably of the type shown in FIGS. 2and 3 and operate to sequentially rollingly engage stationary mountedcommutator bars 84 mounted within housing 65 and which are preferably ofthe type shows in FIGS. 4-4b. The commutator bars are selectivelycoupled to the stator coils 17 through conductors 85, terminalassemblies 66 and 64 and conductors 17a-17b.

A permanent magnet member 86 is secured to rotor shaft 15 and ispositioned in housing 61 immediately adjacent side wall 61a. A secondpermanent magnet member 87 is secured to shaft 69 and is positioned inhousing 65 immediately adjacent side wall 65a and adjacent to member 86.

In operation, the DC source is progressively and sequentially coupled tocoils of the stator-mounted hermetically sealed housing 61 through leads76-77, brushes 74-75, brush rings 71-72, conductors 78-79, contacts80-81, conductive rollers 82-83, commutator bars 84, leads 85-86,terminal assemblies 66 and 64 and conductors 17a-17b. The magneticfields created by coils 17 interact with the fields of the permanentmagnet structure 23 in air gap G to effect rotation. The rotation ofshaft 15 rotates magnetic member 86. The magnetic coupling betweenmembers 86 and 87 imparts rotation to shaft 69 causing the rollercontact assemblies 70--70' and commutator bars 84 to progressivelyswitch DC power to successive stator coils.

Hermetically sealed housing 61 keeps the rotor and stator assemblies,which are practically wear-free, safe from contamination by dust ordirt. Housing 65, however, is designed to be accessible for servicing.Alternatively, housing 65 may comprise a replaceable unit.

While housing 61 is shown as containing the load 88 driven by rotorshaft 15, rotor shaft 15 may extend beyond the left-hand side wall 61bof housing 61 and an appropriate seal may be provided to keep thehousing interior hermetically sealed. The magnetic member 86 may also beeliminated and instead rotor shaft 15 extended beyond the right-handside wall 65a upon providing a similar seal. The magnet member 87 ofshaft 69 may then be eliminated and replaced by keying means on theleft-hand end of shaft 69 for locking shaft 69 to rotate with rotorshaft 15.

FIG. 6 shows a DC motor 100 adapted for use in high speed applications.It comprises a housing 101 which contains bearings 62 and 63 for rotorshaft 15 which has a permanent magnet assembly 23 mounted thereon. Thestator comprises a laminated core 16 having individual laminations 16a.The stator coils 17 are electrically connected to selected commutatorbars 84 by leads 17a-17b.

A pair of brush rings 71 and 72 are mounted upon rotor shaft 15 and arerespectively wipingly engaged by brushes 74 and 75. Leads 76 and 77electrically connect brushes 74 and 75 to opposite polarities of a DCsource.

Roller support assemblies 102 and 103 support roller contact shafts 104and 105 which rotatably mount conductive rollers 106 and 107. Springmounted contacts 108 and 109 are secured to supports 102 and 103 andelectrically connect brush rings 71 and 72 to rollers 106 and 107 byconductors 110 and 111.

The bars 84 of the commutator array are secured in stationary fashionand are selectively connected to end terminals of the stator coils 17 byleads 17a-17b. Conductive rollers 106 and 107, which may be of the typeshown in detail in FIGS. 2 and 2a, rollingly engage the outerperipheries of commutator bars 84.

The diameter of rotor shaft 15 is made as small as practical under thecommutator array while the outer diameter of conductive rollers 106 and107 is made as large as practical. This arrangement, in one preferredembodiment, reduces the angular velocity of the conductive rollers toless than one-half the angular velocity of rotor shaft 17, therebyproviding a motor design which is advantageous for use in applicationsrequiring high speed rotation.

Since the distance traveled by rollers 106 and 107 during one revolutionof rotor shaft 15 is 2π times the outer diameter of the commutator bararray, then for the angular velocity of rollers 106 and 107 to be lessthan the angular velocity of rotor shaft 15 the diameters of rollers 106and 107 must be greater than the outer diameter of the commutator bararray.

The invention disclosed and claimed herein is not limited to thespecific mechanism and techniques herein shown and described sincemodifications will undoubtedly occur to those skilled in the art. Hence,departures may be made from the form of the instant invention withoutdeparting from the principles thereof.

What I claim is:
 1. Switching apparatus for a machine responsive to asource of DC power and having interacting rotor and stator assemblieswhich rotate relative to each other wherein said stator has a windinghaving a plurality of energizeable coils, said coils having an annulararray of commutator bars associated therewith for supplying electricalpower thereto, and said rotor has a magnetic assembly associatedtherewith, said apparatus comprising:a. first and second annular brushrings associated with said rotor; b. first and second brush meansassociated with said stator for respectively coupling said first andsecond brush rings to opposite polarities of said DC source; c. firstroller contact means associated with said rotor and electricallyconnected to said first brush ring for progressively and sequentiallyengaging said commutator bars to thereby momentarily couple saidcommutator bars to said first brush ring; and d. second roller contactmeans associated with said rotor and electrically connected to saidsecond brush ring for progressively and sequentially engaging saidcommutator bars to thereby momentarily couple said commutator bars tosaid second brush ring.
 2. Apparatus according to claim 1 wherein saidfirst and second roller contact means each includes a roller mounted torotate about its axis and to move radially with respect to said rotor.3. Apparatus according to claim 2 wherein said roller is provided withpassageways communicating between a sidewall and the cylindrical surfacethereof for blowing air upon said commutator bars.
 4. Apparatusaccording to claim 1 wherein said rotor has at least one fan bladelocated between said first and second roller contact means for blowingair upon said commutator bars.
 5. Switching apparatus for a machineresponsive to a source of DC power and having interacting rotor andstator assemblies which rotate relative to each other wherein saidstator has a winding having a plurality of energizeable coils, saidcoils having an annular array of commutator bars associated therewithfor supplying electrical power thereto, and said rotor has a pluralityof permanent magnets, said apparatus comprising:a. first and secondannular brush rings associated with said rotor; b. first and secondbrush means associated with said stator for respectively coupling saidfirst and second brush rings to opposite polarities of said DC source;c. first roller contact means associated with said rotor andelectrically connected to said first brush ring for progressively andsequentially engaging said commutator bars to thereby momentarily couplesaid commutator bars to said first brush ring; and d. second rollercontact means associated with said rotor and electrically connected tosaid second brush ring for progressively and sequentially engaging saidcommutator bars to thereby momentarily couple said commutator bars tosaid second brush ring;said first and second roller contact means eachincluding a roller mounted to rotate about its axis and to move radiallywith respect to said rotor, said roller having passageways communicatingbetween a sidewall and the cylindrical surface thereof for blowing airupon said commutator bars.
 6. Switching apparatus for a machineresponsive to a source of DC power and having interacting rotor andstator assemblies which rotate relative to each other wherein saidstator has a winding having a plurality of energizeable coils, saidcoils having an annular array of commutator bars associated therewithfor supplying electrical power thereto, and said rotor has a magneticassembly associated therewith, said apparatus comprising:a. first andsecond annular conductive rings adapted respectively for coupling toopposite polarities of said DC source; b. first roller contact meansassociated with said rotor and electrically connected to said firstconductive ring for progressively and sequentially engaging saidcommutator bars to thereby momentarily couple said commutator bars tosaid first conductive ring; and c. second roller contact meansassociated with said rotor and electrically connected to said secondconductive ring for progressively and sequentially engaging saidcommutator bars to thereby momentarily couple said sommutator bars tosaid second conductive ring,said magnetic assembly comprising a rotorshaft of magnetic material; an array of pole pieces of magnetic materialarranged at spaced intervals about said shaft; a first array of magnetsarranged at spaced intervals about said shaft, radially aligned withsaid array of pole pieces and located between said pole pieces and saidshaft; and a second array of magnets arranged at spaced intervals aboutsaid shaft and positioned between adjacent ones of said pole pieces. 7.Apparatus according to claim 6 wherein said second array of magnets arerare earth magnets.
 8. Apparatus according to claim 6 wherein said firstand second roller contact means each includes a roller mounted to rotateabout its axis and to move radially with rexpect to said rotor, saidroller having passageways communicating between a sidewall and thecylindrical surface thereof for blowing air upon said commutator array.9. Switching apparatus for a machine responsive to a source of DC powerand having interacting rotor and stator assemblies which rotate relativeto each other wherein said stator has a winding having a plurality ofenergizeable coils, said coils having an annular array of commutatorbars associated therewith for supplying electrical power thereto, andsaid rotor has a magnetic assembly associated therewith, said apparatuscomprising:a. first and second annular conductive rings adaptedrespectively for coupling to opposite polarities of said DC source; b.first roller contact means associated with said rotor and electricallyconnected to said first conductive ring for progressively andsequentially engaging said commutator bars to thereby momentarily couplesaid commutator bars to said first conductive ring; and c. second rollercontact means associated with said rotor and electrically connected tosaid second conductive ring for progressively and sequentially engagingsaid commutator bars to thereby momentarily couple said commutator barsto said second conductive ring,said commutator bar array having gapsbetween adjacent commutator bars formed by the side walls of saidadjacent bars, at lease one side wall in each gap having at least thelower portion thereof covered by an insulating material so as to preventparticles collecting in the bottom of said gaps from short-circuitingadjacent commutator bars.
 10. Apparatus according to claim 9 whereinsaid first and second roller contact means each includes a rollermounted to rotate about its axis and to move radially with respect tosaid rotor, said roller having passageways communicating between asidewall and the cylindrical surface thereof for blowing air upon saidcommutator array.
 11. Switching apparatus for a machine responsive to asource of DC power and having interacting rotor and stator assemblieswhich rotate relative to each other wherein said stator has a windinghaving a plurality of energizeable coils, said coils having an annulararray of commutators bars associated therewith for supplying electricalpower thereto, and said rotor has a magnetic assembly associatedtherewith, said apparatus comprising:a. first and second annularconductive rings adapted respectively for coupling to oppositepolarities of said DC source: b. first roller contact means associatedwith said rotor and electrically connected to said first conductive ringfor progressively and sequentially engaging said commutator bars tothereby momentarily couple said commutator bars to said first conductivering; and c. second roller contact means associated with said rotor andelectrically connected to said second conductive ring for progressivelyand sequentially engaging said commutator bars to thereby momentarilycouple said commutator bars to said second conductive ring,said firstand second roller contact means each including a roller for makingrolling contact with the outer periphery of said commutator bar array.12. Apparatus according to claim 11 wherein the diameter of said rolleris larger than the outer diameter of said commutator bar array. 13.Switching apparatus for a machine responsive to a source of DC power andhaving interacting, hermetically sealed rotor and stator assemblieswhich rotate relative to each other wherein one of said assemblies isprovided with a winding having a plurality of energizeable coils and theother of said assemblies has a magnetic assembly associated therewith,said apparatus positioned external to said hermetically sealedassemblies and comprising:a. an annular array of commutator bars fixedrelative to said assembly having said energizeable coils andelectrically connected to said coils for supplying electrical powerthereto; b. first and second annular conductive rings adaptedrespectively for coupling to opposite polarities of said DC source; c.first and second roller contact means fixed relative to said magneticassembly and electrically connected respectively to said first andsecond conductive rings; and d. means for coupling the relative rotationof said hermetically sealed rotor and stator assemblies to effectrelative rotation between said first and second roller contact means andsaid commutator bars so that said first and second roller contact meansprogressively and sequentially engage said commutator bars tomomentarily couple said commutator bars to said first and secondconductive rings.
 14. Apparatus according to claim 13 wherein said firstand second roller contact means each includes a roller mounted to rotateabout its axis and to move radially with respect to said rotor, saidroller having passageways communicating between a sidewall and thecylindrical surface thereof for blowing air upon said commutator array.15. Switching apparatus according to claim 13 wherein said winding isassociated with said stator and said magnetic assembly is associatedwith said rotor.
 16. Switching apparatus according to claim 15 furtherincluding:a. a first rotor shaft for supporting said hermetically sealedrotor assembly; b. a second rotor shaft for supporting said first andsecond roller contact means; c. hermetically sealed barrier meanspositioned between said first and second rotor shafts for isolating saidhermetically sealed rotor and stator assemblies from said switchingapparatus; d. first and second magnet means mounted respectively on saidfirst and second rotor shafts and positioned proximate each other forcoupling the relative rotation of said rotor and stator assemblies tosaid roller contact means and said commutator bars.
 17. Switchingapparatus for a machine responsive to a source of DC power and havinginteracting rotor and stator assemblies which rotate relative to eachother wherein one of said assemblies is provided with a winding having aplurality of energizeable coils, said coils having an annular array ofcommutator bars associated therewith for supplying electrical powerthereto, and the other of said assemblies is provided with a magneticassembly, said apparatus comprising:a. first and second annularconductive rings adapted respectively for coupling to oppositepolarities of said DC source; b. first roller contact means associatedwith said other assembly and electrically connected to said firstconductive ring for progressively and sequentially engaging saidcommutator bars to thereby momentarily couple said commutator bars tosaid first conductive ring; and c. second roller contact meansassociated with said other assembly and electrically connected to saidconductive ring for progressively and sequentially engaging saidcommutator bars to thereby monentarily couple said commutator bars tosaid second conductive ring.
 18. Apparatus according to claim 17 whereinsaid commutator bars are skewed so that when said first and secondroller contact means engage said commutator bars they effect amake-before-break contact sequence.
 19. Apparatus according to claim 17wherein said first and second roller contact means each includes aroller mounted to rotate about its axis and to move radially withrespect to said rotor, said roller having passageways communicatingbetween a sidewall and the cylindrical surface thereof for blowing airupon said commutator array.
 20. Switching apparatus for a machineresponsive to a source of DC power and having interacting rotor andstator assemblies which rotate relative to each other wherein one ofsaid assemblies is provided with a winding having a plurality ofenergizeable coils, said coils having an annular array of commutatorbars associated therewith for supplying electrical power thereto, andthe other of said assemblies is provided with a magnetic assembly, saidapparatus comprising:first and second roller contact means associatedwith said other assembly and adapted respectively for connection toopposite polarities of said DC source and to progressively andsequentially engage said commutator bars to momentarily couple saidcommutator bars to opposite polarities of said DC source.